1. Field of the Invention
The present invention relates to an apparatus and method for repeatedly acquiring a monitoring image that is used to monitor a change in a given region of interest of a subject.
2. Description of the Related Art
As one of the fields of application of the magnetic resonance imaging (MRI) method, there is magnetic resonance angiography (MRA). MRA can be realized by MRI through the use of various fundamental contrast generation principles such as the time-of-flight (TOF) effect or phase shift effect. One extensively used method (which will be referred to as contrast MRA hereinafter) is to rapidly inject a contrast medium having a longitudinal relaxation time (T1) reduction effect into the human body and imaging blood having this contrast medium mixed therein. Using contrast MRA makes it possible to obtain images of a main artery, a renal artery, or a blood vascular system including a neck region, a head region, a foot region, and others.
Imaging time in contrast MRA is generally in the range of approximately several seconds to several tens of seconds. Therefore, only one imaging operation may be performed per contrast medium injection operation.
However, the contrast medium is usually injected at a point distant from the region considered an imaging target (which will be referred to as the target region hereinafter). Therefore, the time at which the contrast medium reaches the target region to enable acquisition of an image having good contrast lags the contrast medium injection time. Further, the delay is dependent on, for example, the cardiac rate, blood pressure, or blood flow rate of the subject, and hence it is not fixed.
Under the circumstances, in contrast MRA, appropriately setting imaging timing is important, and ingenuity in realizing this setting has been conventionally exercised as follows.
For example, firstly, there is known a technology that continuously acquires a magnetic resonance signal from a limited monitoring region (for example, the inside of a main artery on the upstream side of a target region) close to the target region prior to the contrast MRA imaging, presents an operator with a change in signal intensity with time, and starts imaging in synchronization with a timing at which the signal intensity is increased to a threshold value or above (see PCT National Publication No. 2000-511789).
A second technology is suggested as an alternative of the first technology, and this technology adopts fluoroscopy using a two-dimensional imaging method to monitor a relatively wide range and directly provides a state of movement of a contrast medium as a change in image signal (see Radiology, Vol. 205, p. 137 [1997]).
In this second technology, since a wide range can be continuously imaged, how the contrast medium flows in a subject can be displayed in real time. That is, for example, how the contrast medium injected from an upper arm or a subject flows through a lung, an atrium, and a main artery can be displayed in real time. An operator chooses a proper timing at which the contrast medium reaches a target region based on this display and instructs starting the contrast MRA imaging. The contrast MRA imaging is started at a timing that the contrast medium reaches an object region determined on an upstream side of the target region.
Thirdly, there is known a technology that performs subtraction processing with respect to an image signal obtained in a second conventional example or maximum value projection processing of multi-slice data in order to further clearly display how a contrast medium moves (see JP-A 2003-235827 [KOKAI]).
It should be noted that setting a region of interest (ROI) required to measure a signal value on a two-dimensional image is known (see JP-A H10-192252 [KOKAI]), but specification of the position of this ROI by an operator is not performed before the contrast medium reaches a target region.
However, in the first technology, an increase in signal intensity is not sufficiently ascertained in some cases as described in Radiology, Vol. 203, p. 275 (1997). It is considered that this increase cannot be ascertained because a blood flow signal cannot be satisfactorily observed due to breathing or body movements in a monitoring region. Further, with this first technology, a monitoring region having a small volume must be appropriately positioned, and there is the inconvenience that operation is complicated.
On the other hand, with the second and third technologies, it is hard for an operator lacking sufficient experience in contrast MRA inspections to accurately ascertain that the contrast medium has reached a target region, and it is possible that the contrast MRA imaging is not started at the right time. This is because, in a fluoroscopy image, the signal is weak at positions except where the contrast medium has entered, and hence the position of a target region or an object region is hard to ascertain.